Hi, Brian.
I have no idea how or to what extent terrain affects a vertically
polarized signal. I am, however, pretty certain that if it does at all
it would be worth more than 1 db. That's just the nature of proximity
effects in general. I suspect that one possible effect of sloping
ground for a vertical antenna might be to move the lossy environment to
where it has less effect on the signal (i.e., lower), and that might be
more significant with poor soil conductivity than with good
conductivity. It seems reasonable to me to imagine that the pattern for
sloping ground would shift at least to some minor extent toward that of
a free space pattern, but I'm pretty sure that ON4UN and others have far
more insight into this than I do.
I've always wished we had a version of HFTA that handled vertically
polarized antennas ... that would probably be enlightening. I've also
tried on occasion to use EZNEC+ for the same purpose since it allows you
to specify at least two different zones around the antenna, but I didn't
have much success with that.
I'm one of those who had quite good luck with roof mounted verticals,
but it's hard to say whether any improvement (if there was one) was the
result of distance from lossy ground or simply the ability to shoot over
lossy surrounding structures like trees and houses. I do think it is
more than "folklore" that elevated radials have benefits over in-ground
radials unless the in-ground radial system is reasonably extensive.
There have been some pretty decent studies on that.
But again, it is important to keep separate the effect of radials on
feedpoint return loss from the effect of ground on radiation pattern.
Radials reduce feedpoint loss but have minimal effect on radiation
pattern unless you are able to go out several wavelengths. On the other
hand, the conductivity of the ground has much less effect on return loss
than a decent radial system, but it is the ONLY determinant of far field
radiation pattern (other than whatever effect the terrain profile itself
may have).
73,
Dave AB7E
On 7/13/2016 3:46 AM, brian wrote:
Hi Dave.
Care to comment on how much benefit vertically polarized antennas
might gain from terrain sloping away from vertical?
For horizontally polarized antennas, where ground reflection gain is
up to 6 dB, the sloping terrain can lower the effective take off angle
a lot- 10's of degrees. There was a program called YTAD that
estimated this effect in one dimension. It's results were quite
enlightening.
Vertical antennas have no ground reflection gain. Would one then
expect sloping terrain NOT to alter their already low take off angle
much? On the other hand, folklore seems to indicate a benefit of roof
top verticals with their "elevated" radials over ground mounted
verticals with elevated radials/or in ground radials.
73 de Brian/K3KO
On 7/13/2016 8:49 AM, David Gilbert wrote:
Agree on the coax losses, but totally disagree on both of your other
comments:
1. Elevated radials will ONLY help reduce near field ground losses ...
they will do nothing to help radiation pattern. The effects of ground
conductivity determine far field pattern (given a particular profile) no
matter what kind of radials he uses.
2. If there is any gain benefit from the terrain profile at all, it
will be much more than 1 db.
Dave AB7E
On 7/12/2016 3:49 PM, Craig Smith wrote:
Ted …
I think you are overstating the coax losses. Even stock RG-11 should
be perhaps 0.3 dB/100ft on 80 meters - around 1.5 dB for the 500 ft.
run.
Even so, I would probably gravitate toward the closer location. With
the elevated radials, the effects of the ground conductivity should be
minimal. Not sure if ON4UNs data assume elevated radials or many
on-ground radials. It could be that his estimate of the sloping
ground advantage is for the later. With the closer location, you
will have perhaps 1 dB stronger signal in all directions because of
the lower feedline loss.
73 Craig AC0DS
_
a
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